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Researchers from MIT and GM propose a tool for better estimating secondary mass savings potential; maximizing vehicle mass savings for greater fuel economy

17 February 2012

Alonso1
More accurately estimating SMS can maximize mass savings, thereby increasing fuel economy. Credit: ACS, Alonso et al. Click to enlarge.

Researchers from MIT and GM have developed a tool for estimating secondary mass savings potential early in the vehicle design process. Using the tool early in the process—before subsystems become locked in—maximizes mass savings, they report in a paper published in the ACS journal Environmental Science & Technology.

Secondary mass savings are mass reductions that may be achieved in supporting (load-bearing) vehicle parts when the gross vehicle mass (GVM) is reduced. Mass decompounding is the process by which it is possible to identify further reductions when secondary mass savings result in further reduction of GVM. Maximizing secondary mass savings (SMS) is a key tool for maximizing vehicle fuel economy, they note, but can be difficult to achieve given the current design process.

Various engineering and design approaches can be used to reduce mass, including materials substitution, novel processing techniques, and design optimization. A number of authors have noted that all of these mass saving approaches are challenging to implement because it is difficult to estimate their impact on costs and on load path management.

An additional challenge emerges from the nature of the vehicle development process, which is time-constrained and in which subsystems are designed concurrently. As such, to maximize mass savings it is necessary to have sound estimates of the impact of any mass solution, and those estimates must be available early, before key design details are locked in. This paper will focus solely on one aspect of addressing this information challenge for mass reduction engineering: estimating secondary mass saving (SMS) for proposed mass solutions. With a better understanding of SMS potential, vehicle designers should be better able to set early mass targets and analysts should be able to develop better estimates of the life-cycle cost, fuel use, and environmental impact of mass changes.

—Alonso et al.

SMS essentially reflects that the size (and mass) of some components is at least partially determined by the need to bear the mass of other components—i.e., if vehicle mass decreases, the mass of some components can also decrease. SMS, the authors note, can increase the attractiveness of any proposed mass reduction by increasing the realized mass change associated with any significant primary reduction.

The implications of this are simple: if vehicle manufacturers underestimate SMS, they are more likely to think that any given mass reduction solution has too little impact, is too costly, or both.

Implementing (and therefore estimating) SMS is challenging because of fundamental limitations related to the vehicle development process, use of carryover parts, and component sharing. The vehicle development process lasts 50−60 months before vehicle launch, and to accommodate the complexity and cost of change, it involves locking in certain vehicle subsystem designs as the process progresses. Once a subsystem is locked, its components are no longer available to realize SMS.

—Alonso et al.

In their paper, Alonso et al. work to improve empirical estimation of SMS potential by:

  1. providing a formal statement of the analytical method;
  2. developing the methods for quantifying uncertainty in the estimation of SMS potential;
  3. characterizing the inherent upper-bound bias of this method; and
  4. quantifying the importance of expert classification of data at the component level for managing the impact of mass-independent effects on the analysis.

The team estimated the potential for SMS in current passenger vehicles with an empirical model using engineering analysis of vehicle components to determine mass-dependency. They grouped the identified mass-dependent components into subsystems, and performed linear regression on subsystem mass as a function of GVM.

A Monte Carlo simulation determined the mean and 5th and 95th percentiles for the SMS potential per kilogram of primary mass saved.

The model projects that the mean theoretical secondary mass savings potential is 0.95 kg for every 1 kg of primary mass saved, with the 5th percentile at 0.77 kg/kg when all components are available for redesign.

Using the model to explore an alternative scenario where realistic manufacturing and design limitations were implemented, they found four key subsystems (of 13 total) were locked-in. This reduced the SMS potential to a mean of 0.12 kg/kg with a 5th percentile of 0.1 kg/kg. This, they concluded, clearly showed that targets need to be established before subsystems become locked in to maximize mass reductions.

In the end, the analyses in this paper suggest that on average it may be possible to realize an entire additional kilogram removed (and almost certainly, in 95% of cases, 0.7 kg) for every kilogram removed through more novel methods. This is much larger than previously reported expert based-estimates which commonly center around 0.5 kg/kg. Those additional kilograms could transform the impact of investments in mass saving technologies. Consider that a SMS potential around 1 changes a 10% primary mass reduction...into a 20% reduction if load path and customer requirements remain unchanged. On average, that mass savings translates into a 10% savings in fuel economy.

If realized in the US fleet alone, a 10% reduction translates into savings of approximately 0.9 million barrels of petroleum per day (currently the US uses about 19 million barrels per day), or almost 200 million metric tons of carbon dioxide equivalent per year. Given the direct tie between mass and fuel use, challenging the industry to remove this additional mass is an important step toward a more sustainable automotive fleet.

—Alonso et al.

Resources

  • Elisa Alonso, Theresa M. Lee, Catarina Bjelkengren, Richard Roth, and Randolph E. Kirchain (2012) Evaluating the Potential for Secondary Mass Savings in Vehicle Lightweighting. Environmental Science & Technology doi: 10.1021/es202938m

February 17, 2012 in Fuel Efficiency, Weight reduction | Permalink | Comments (9) | TrackBack (0)

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Comments

After many foolish decades of 'Bigger is Better' it is about time to have a serious look at how to reduce private vehicles weight by 50% of so, specially for future electrified vehicles to get more range with smaller batteries. It is rather foolish to use a 6000 lbs vehicle to go to work downtown.

Lighter cars can be roomy and safe. Saturn used space frames and polymer panels to take hundreds of pounds off of their compact sedans versus the competition. There was mention of the panel gaps for expansion with heat, but that was a small price to pay for better mileage and in the case of EVs better range.

We can do this, but public perception is heavier is safer. True, in a head on collision between two cars, the heavier one wins, but not in a collision with a bridge abutment. Safety can be designed in without adding weight, they have been doing this in race cars for decades and saving lives in the process.

The majority of us will believe just about anything if it is repeated often enough. Smart Ads people know that and are using it to brain wash the unaware and vulnerable.

Tobacco industries used that approach with great success for decades. At one point, almost 100% of targeted group were smoking. Sweet energy drinks and other harmful drinks/foods are being pushed on the most vulnerable the same way.

The local Big-3 car industries used a very similar approach to convince buyers that a private vehicle should weight more, be more powerful, should be wider and longer every year. They even lowered the quality so should change vehicle more often. They succeeded up to 8000 lbs Hummers and the current financial recession.

GM, Ford and Chrysler were all addicted to the high profit margins of large trucks and SUVs, so that was fine for them until $4 gasoline hit in 2008.

The SUV market did not even exist until GM and Ford watched Jeep in the 80s, then the Ford Explorer took off in the 1990s and the race was on to see who could sell the most SUVs converted from truck chassis. Now it is crossovers from car chassis.

People in the U.S. just think they NEED a large vehicle that can haul 6 kids and a boat. 360 days per year they do not, but give them the image of those other 5 days and they will drive a big gas guzzling vehicle the other 360 for those 5.

With taxes and regulations very little industry can survive in the US or Europe.

The cheapest, fastest way to reduce CO2 production from automobiles and improve efficiency is to reduce motorway speeds.

The use of hydraulic hybrids with regeneration is the next way. No battery can match the peak power of compressed air in a tank.

Lithium batteries are a bad engineering choice right now because they are too expensive.

Large batteries for long range are a bad engineering choice right now because of high cost.

The original 2CV with about ten real horse-power (CV) was adequate for most transportation purposes, and when combined with hydraulic regeneration is still better and more efficient, but will block traffic on motorways.

High power electric motors are an engineering waste for most transportation needs; as are their big electronic drives because they are too expensive. ..HG..

HG...I used an old Two Chevaux (2CV) in Africa for almost 2 years. It was real fun to drive on unpaved very rough roads. It would dance in and out of large deep holes with ease. Changing gear was somewhat approximative. That little monster could go a long way on very little gas. The starter eventually gave up but I had no problem getting a free push by local kids.

Will the 'Midi' compressed air car be built soon in India?

BEVs will make more sense when batteries have improved 300+% and their price is down to $150/KWh or even less. That may take another 10+ years or so but it will come. Meanwhile, subsidies will be required to offset some of the extra cost. Toyota is working on a multi-fuel mini turbine-generator that could find application in mini (2CV Style) PHEVs soon. It is so small and light that it could also be used on 2 or 3-wheel e-bikes to extend their e-range with smaller batteries. Most PHEVs do not need large, heavy, expensive 150 HP on-board power generators. An ultra light 10 KW unit could do the job on much lighter future vehicles.

Here's an idea that could show huge weightsavings if applied to the structural members of cars; http://www.delta7bikes.com/isotruss-lightweight-bikes.htm

I honestly couldn't care less about what GM and MIT think about reducing the weight of vehicles. Fact of the matter is bigger IS better --- I will be owning & buying solid, heavy, full size trucks with big block V-8's the rest of my life.

Unfortunately, the industrialized free enterprise world has created many millions ill conceived minds. It may take a few generations and/or deep long lasting recessions to fix them.

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